This invention relates in general to corona charging devices for use in electrostatographic reproduction machines, and more particularly to a control grid electrode for connection to a corona charging device used in such machines.
In typical commercial reproduction apparatus (electrographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
In such reproduction apparatus, the dielectric support member is typically initially charged by spraying its surface with ions produced by a corona charging device. The corona charging device has a plurality of fine wires positioned transversely to the direction of motion of the dielectric support member and energized by a high voltage power supply. Corona emission from the wires ionizes air molecules, which are drawn to the surface of the dielectric support member as the dielectric support member moves past the wires. The quality of the developed image, especially that of pictorial images, is very dependent on the uniformity and consistency of the initial charge on the surface of the dielectric support member.
To improve the uniformity and consistency of the initial dielectric support member surface charge, it is known to provide an electrically biased control grid between the corona wires and the dielectric support member. The control grid includes a series of parallel wires or narrow metal strips closely spaced to each other and positioned transverse to the direction of motion of the dielectric support member. The spacing of the control grid wires to each other and to the dielectric support member surface, and the electrical bias voltage applied to the control grid wires, are the parameters that control the amount and spatial uniformity of the ionic charge deposited onto the surface of the dielectric support member. The surface voltage potential created on the dielectric support member is directly related to the bias voltage applied to the control grid wires.
The surface voltage created on the dielectric support member also depends upon the velocity at which such member moves past the corona charging device. For slower reproduction apparatus in which the dielectric support member velocity might only be a few inches per second, a corona charger with only one corona wire may be adequate. However, for higher speed reproduction apparatus in which the dielectric support member velocity might be 15-20 inches per second or higher, a corona charger with a plurality of corona wires may be required to achieve the required surface charge amount and uniformity.
Optimum charging efficiency, as well as uniformity and consistency of the deposited charge, is achieved when the control grid is closely and uniformly spaced from the surface of the dielectric support member. If the dielectric support member is curvilinear shaped and a corona charger with a plurality of corona wires is required, this would require that the control grid be provided with a curvature that matched the curvature of the dielectric support member. It is however very difficult to give this curvature to the control grid and still maintain the uniform spacing of the control grid from the dielectric support member surface because the control grid is typically made up of a plurality of closely spaced, thin wires. Tensioning the wires of a curved control grid so as to keep them uniformly spaced to each other and to the surface of the dielectric support member is very difficult.
U.S. Pat. No. 5,206,784 (issued Apr. 27, 1993, in the name of Kimiwada, et al.) addresses uniformly spacing the control grid from a cylindrically shaped member by dividing the control grid into a plurality of separate and independent planar grids equally spaced from the cylindrical member. The use of separate and independent planar grid sections is an effective compromise to a single piece curved grid, but the charger is costly to manufacture and difficult to assemble and to service if the grid sections become contaminated and must be replaced. It is difficult and time consuming to precisely position and equally tension the separate sections relative to each other as is required to achieve optimum dielectric support member surface charge uniformity and consistency.
In view of the above, it is the object of the present invention to provide a control grid electrode for a corona charging device for charging a curvilinear shaped dielectric support member with improved charging uniformity and consistency and with lower assembly and service cost. The single piece control grid is formed into a plurality of planar segments angularly oriented with respect to each other so that each segment is substantially equally spaced from the curvilinear shaped dielectric support member. Attaching this single piece control grid to the corona charging device during initial assembly and service replacement is easier than with separate segments. Further, maintaining the required spatial relationship between each segment and between the segments and the surface of the curvilinear shaped dielectric support member is facilitated by this arrangement. Thus assembly, operation, and service cost are optimized.
The invention, and its objects and advantages, will become apparent in the detailed description of the preferred embodiment presented below.